Electronic circuits and systems often include electronic switches. An electronic switch can be used to transmit an analog signal to a circuit path or to prevent an analog signal from being sent to a circuit path. Such a switch is sometimes referred to as an analog switch or a pass switch to differentiate this type of switch from a digital switch that changes its output state in response to an input, but does not pass a received signal. Switches are designed to respond to a nominal control voltage that enables an “on” state of the switch. The control voltage can determine an expected ON-resistance. However, physical properties of the switch and ambient conditions can cause the ON-resistance of the switch to deviate from the expected ON-resistance even when the control voltage is maintained at the nominal value. Deviation of the ON-resistance of the switch from the expected value can distort the signal as it passes through the switch. Factors that can contribute to the ON-resistance of the switch deviating from an expected value when a nominal control voltage is maintained at the switch include, but are not limited to, process variations between switches or temperature variation in a particular switch.
FIG. 1 illustrates generally relationships 100 between switch ON-resistance 110 (ohms) over a range of analog signal voltages 111 applied to a fast process-corner switch and a slow process-corner switch under different supply voltage and temperature conditions. Process-corner speed is related to the threshold voltage of a switch, such as a MOS transistor switch. A fast process-corner switch generally includes a lower threshold voltage than a slow process-corner switch. Various factors can contribute to threshold voltage variations in switches intended to be identical, including process variations.
The example of FIG. 1 illustrates generally, for the fast process-corner switch, plots 102, 105, 106, 108 and, for the slow process-corner switch, plots 101, 103, 104, 106. The vertical axis 110 indicates resistance (ohms) between the switched terminals of each switch, for example, the resistance between the drain and source of a switch including a MOS passgate. The horizontal axis 111 indicates the voltage applied to the switched terminals of the switch.
For the fast process-corner switch, plots 102, 106 are characterized by a device having a 3.0-volt supply voltage, and plots 105, 108 are characterized by a device having a 4.4-volt supply voltage. Plots 106, 108 are characterized by a device subjected to a temperature of about −40 degrees Celsius, and plots 102, 105 are characterized by a device subjected to a temperature of about 80 degrees Celsius.
Similarly, for the slow process-corner switch, plots 101, 103 are characterized by a device having a 3.0-volt supply voltage, and plots 104, 107 are characterized by a device having a 4.4-volt supply voltage. Plots 103, 107 are characterized by a device subjected to a temperature of about −40 degrees Celsius, and plots 101, 104 are characterized by a device subjected to a temperature of 80 degrees Celsius.
The ON-resistance of the various measurement conditions ranges over about 9.7 ohms when the applied signal voltage is 0.4 volts, at 112, and over about 10.3 ohms when the switched voltage is about 0.5 volts, at 113. For some applications and signal protocols, such a range of ON-resistance may result in performance discrepancies of a particular switch, or for switches with processing variations, that are not acceptable.